1. Field of the Invention
The present invention relates to communications amplifiers, and more particularly to remotely powered intermediate amplifiers for communications transmission incorporating two conductor coaxial transmission lines.
2. The Prior Art
Intermediate amplifier stations are known for wide-band transmission systems, as for example, carrier-frequency amplifiers, which are remotely powered by DC potential conveyed to the amplifier over the transmission line. When a two conductor transmission line is employed, the transmission line must carry the DC voltage and the AC signals, and so the reference potential of the amplifier cannot be connected directly to the transmission line. When coaxial cables are employed, generally a housing is provided and the outer conductors of the coaxial cables are connected to the housing. Between the amplifier reference potential and the housing a low impedance alternating current connection must be provided, for passing AC signals in the transmission band. This connection is typically produced by means of a blocking capacitor.
The use of a blocking capacitor produces several disadvantages. The capacitor must be capable of blocking relatively high voltages, which causes it to have at least a minimum size, which in turn leads to an inductance which is not negligible at high frequencies. This also leads to parallel resonances at certain frequencies which is extremely undesirable. Moreover, the AC signal in flowing through the capacitor causes a voltage drop across the capacitor, and this voltage produces a current in the input circuit so that a feedback takes place from the output to the input. This feedback produces changes in the frequency response to the amplifier and can lead to instability.
It is therefore desirable that an arrangement be provided whereby the disadvantages of the prior art arrangements can be overcome.